A condensing boiler system will cause the steam of combustion of a fuel to condense to liquid water and will also collect the latent heat of vaporisation of the steam and recycle this heat into the boiler system and thus increase the thermal efficiency of the boiler system.
Thus it has previously been proposed to recover heat from the flue gases of a water boiler in earlier UK Patent Application No. 0107963.1, using a secondary heat exchanger, which raised the gross efficiency of a boiler from about 89.5% to about 92.5%.
The following points have been noted in relation to water boilers:    1. The flue gas temperature from a boiler without a condensing secondary exchanger is well above 100° C., typically in the range 150°–250° C., and it is quite certain that such a boiler recovers no latent heat from steam because the flue gas temperature is too high for any condensation to take place. If then, such a boiler operates with a gross efficiency of nearly 90%, then it is possible to conclude that all the steam latent heat is still trapped in the flue gases effluent from the boiler, and will escape up the flue.    2. The same boiler, fitted with a stainless steel (or other suitable metal) secondary heat exchanger as previously proposed, will demonstrably operate with a gross efficiency of about 93%, an efficiency gain of just 3%. This secondary exchanger, although fitted with a water condensate overflow tube, has been found to provide only a liter or so of water over an operating time of several hours. Therefore the secondary exchanger (condenser) described, although condensing some water and therefore correctly termed a condensing heat exchanger, is only condensing a small proportion of the steam, and a proper description would be a partially condensing heat exchanger because in no way is the volume of water collected representative of the volume of steam generated by the burning fuel, but is only a fraction thereof.    3. As confirmation of (2) above, large quantities of steam can be seen issuing to atmosphere from the secondary exchanger previously proposed, and this shows that at any rate some steam escapes the secondary exchanger and latent heat escapes with the steam.    4. In a typical example of a kerosene based fuel, the percentage of hydrogen in the fuel by weight is about 13.74%. Thus it can be shown that burning 1 US gallon per hour of the fuel will produce 3.6 kilograms of water as steam per hour. Moreover, the specific latent heat of steam at 100° C. is 2.26 megajoules per kilogram, or almost 6% of the total calorific value of the fuel, and so far only a fraction of this has been recovered by secondary heat exchangers as proposed. For total latent heat recovery to take place, it follows that about 3.6 kilograms of water could (and should) be recovered. In practice perhaps as little as 10% is actually recovered.
Therefore although a 3% efficiency gain has been achieved, the principal amount of energy recovered by the previously proposed secondary heat exchanger is due to reducing the temperature of boiler exhaust gases. The bulk of the steam and its energy escapes to atmosphere, as evidenced by the small volume of condensed water collected    5. The dew point of the water vapour component in the flue gas of a typical domestic boiler is about 50°–55° C. It is quite impossible to fully condense this water and to capture the energy of condensation unless the medium used to cool the gases is below, and ideally considerably below, the dew point. The temperature of the return water from the radiators previously proposed to cool the flue gases is at about 60° C., and therefore is unavailable to cool the flue gases below the dew point.    6. It is not considered sensible to reduce the water flow through the radiators so that the return temperature is sufficiently low (e.g. 25°–30° C.) as to allow this water to be employed in the heat exchanger to effect full condensation of the water vapour in the still hot gases.